1. Field of the Invention
The present invention generally relates to control bus structures, and more particularly to a multi-purpose bi-directional control bus.
2. Description of the Related Art
Control bus structures in hardware have long implemented dedicated unidirectional control signals. Dedicated unidirectional control signals have relied upon point-to-point connections or wiring to form fixed signal paths. While some control signals are only originated by a target or slave device and terminated at an initiator or master device, other control signals are only originated by an initiator or master device and terminated at a target or slave device. Most control signals today fit into one of these two categories. One class of control signals outside of these two categories, however, is control signals originated by initiator devices and terminated at target devices and other initiator devices.
The table below lists some of the control signals associated with the Am486(copyright)DX microprocessor family, along with the description and source device for each listed control signal.
As indicated by the table, each of the signals above is either a unidirectional control signal provided by an initiator device to a target device or a unidirectional control signal provided by a target device to an initiator device.
Due to the specific nature of unidirectional control signals, such control signals have been limited not only in the type of information they carry but also by when they may be utilized. Most unidirectional control signals have only been utilized for a small percentage of a bus cycle or transaction. Many unidirectional control signals therefore have been unutilized during majority of a bus cycle.
Coupling multiple initiator devices to control bus structures has depended upon use of glue logic. Each initiator device has been associated with its own set of dedicated unidirectional control signals. Glue logic has typically served to combine the same control signals from the initiator devices into a common control signal. Multiple sets of glue logic thus have been necessary to generate a common control signal for each dedicated unidirectional control signal shared by any initiator devices.
Briefly, according to the present invention, a computing or processor-based system, such as a microcontroller, provides a multi-purpose bi-directional control bus between system devices. The control bus can be shared by multiple initiator devices and multiple target devices. An initiator device can transmit a token representing a control command only designated by an initiator device via the control bus to a target device, and a target device can transmit a token representing a control command only originated by a target device via the control bus to an initiator device. A token can represent a number of control commands. A 4-bit control bus, for example, supports 16 possible tokens. Both initiator devices and target devices support encode logic and decode logic to interpret tokens.
The control bus provides bi-directional multi-purpose control lines for carrying tokens. Any control line can provide a token from an initiator device to a target device and provide a token from a target device to an initiator device. As compared to conventional unidirectional control signals, the control bus reduces pincount and point-to-point connections between system devices. Control signals which once were handled using separate control lines can now be handled on a single bi-directional control line. The control bus also provides improved utilization of control lines since its bi-directional control lines may be used for any phases of a bus cycle or transaction.